Semiconductor device

ABSTRACT

A semiconductor device includes an isolation feature in a substrate. The semiconductor device further includes a first source/drain feature in the substrate, wherein a first side of the first source/drain feature contacts the isolation feature, and the first source/drain feature exposes a portion of the isolation feature below a top surface of the substrate. The semiconductor device further includes a silicide layer over the first source/drain feature. The semiconductor device further includes a dielectric layer along the exposed portion of the isolation feature below the top surface of the substrate, wherein the dielectric layer contacts the silicide layer. The semiconductor device further includes a second source/drain feature in the substrate on an opposite side of a gate stack from the first source/drain feature, wherein the second source/drain feature has a substantially uniform thickness.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.14/268,353, filed May 2, 2014, which is a continuation of U.S.application Ser. No. 13/461,532, filed May 1, 2012, now U.S. Pat. No.8,735,255, issued May 27, 2014, which are incorporated by referenceherein in their entireties.

BACKGROUND

The recent trend in miniaturizing integrated circuits (ICs) has resultedin smaller devices which consume less power, yet provide morefunctionally at higher speeds than before. The miniaturization processat advanced process nodes has also posed numerous technical challengesrarely observed at low process nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments are illustrated by way of example, and not bylimitation, in the figures of the accompanying drawings, whereinelements having the same reference numeral designations represent likeelements throughout. The drawings are not to scale, unless otherwisedisclosed.

FIGS. 1A-1G are schematic cross-sectional views of a semiconductordevice in various manufacturing operations in accordance with someembodiments.

FIG. 2 is a flow chart of a method of manufacturing a semiconductordevice in accordance with some embodiments.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. The inventiveconcept may, however, be embodied in many different forms and should notbe construed as being limited to the embodiments set forth herein;rather, these embodiments are provided so that this description will bethorough and complete, and will fully convey the inventive concept tothose of ordinary skill in the art. It will be apparent, however, thatone or more embodiments may be practiced without these specific details.

The drawings are not drawn to scale, and include certain features thatare exaggerated for clarity. Like reference numerals in the drawingsdenote like elements. The elements and regions illustrated in thefigures are schematic in nature, and thus relative sizes or intervalsillustrated in the figures are not intended to limit the scope of theinventive concept.

In some embodiments, a Si-containing layer is formed over a source/drainfeature before performing a silicidation process. The Si-containinglayer reacts with a metal layer during the silicidation process to forma metal silicide layer.

FIGS. 1A-1G are schematic cross-sectional views of a semiconductordevice 100 at various manufacturing stages in accordance with someembodiments. In one or more embodiments, the semiconductor device 100comprises active components such as field effect transistors (FET),complementary metal-oxide-semiconductor (CMOS) transistors,metal-oxide-semiconductor field effect transistors (MOSFETs), highvoltage transistors, high frequency transistors, and combinationsthereof. In one or more embodiments, the semiconductor device 100comprises passive components, such as resistors, capacitors, inductors,and fuses. In the description below, one or more methods ofmanufacturing a p-channel metal-oxide semiconductor (PMOS) device and/oran n-channel metal-oxide semiconductor (NMOS) device will be described.However, further embodiments are applicable to other types ofsemiconductor devices or components, especially where a metal silicidelayer is to be formed.

In the operation in FIG. 1A, a substrate 110 is formed with a gate stack120 and an isolation feature 130. In some embodiments, the substrate 110comprises an elementary semiconductor, a compound semiconductor, analloy semiconductor, or combinations thereof. Examples of the elementarysemiconductor include, but are not limited to, silicon and germanium.Examples of a compound semiconductor include, but are not limited to,silicon carbide, gallium arsenic, gallium phosphide, indium phosphide,indium arsenide, and indium antimonide. Examples of the alloysemiconductor include, but are not limited to, SiGe, GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and GaInAsP. Other semiconductor materialsincluding group III, group IV, and group V elements are used in someembodiments. In one or more embodiments, the substrate 110 comprises asemiconductor on insulator (SOI), a doped epitaxial layer, a gradientsemiconductor layer, and/or a stacked semiconductor structure with onesemiconductor layer (e.g., Si) overlying another semiconductor layer(e.g., Ge) of a different type.

In some embodiments, the substrate 110 comprises one or more dopedregions. A p-type doped region comprises p-type dopants, such as boronor BF2. An n-type doped region comprises n-type dopants, such asphosphorus or arsenic. In one or more embodiments, doped regions areformed directly on the substrate 110, in a P-well structure, a N-wellstructure, a dual-well structure, or a raised structure. In one or moreembodiments, PMOS devices and NMOS devices are formed in differentdevice regions with corresponding doped regions. The isolation feature130 is formed in the substrate 110 to isolate various regions of thesubstrate 110, such as device regions for PMOS devices from deviceregions for NMOS devices.

The gate stack 120 comprises a gate dielectric layer 122 and a gateelectrode 124. Other layers are included in some embodiments. The gatestack 120 is formed by chemical vapor deposition (CVD), physical vapordeposition (PVD), atomic layer deposition (ALD), high density plasma CVD(HDPCVD), plating, other suitable methods, or combinations thereof.

The gate dielectric layer 122 is formed over the substrate 110 andcomprises a dielectric material, a high-k dielectric material, othersuitable dielectric material, or combinations thereof. Examples of adielectric material include, but are not limited to, silicon oxide,silicon oxynitride, silicon nitride, or combinations thereof. Examplesof high-k dielectric materials include, but are not limited to, HfO₂,HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, or combinations thereof. In someembodiments, the gate dielectric layer 122 comprises a multilayerstructure. For example, the gate dielectric layer 122 comprises aninterfacial layer, and a high-k dielectric material layer formed on theinterfacial layer. An exemplary interfacial layer is a grown siliconoxide layer formed by a thermal process or ALD process.

The gate electrode 124 is formed over the gate dielectric layer 122. Inone or more embodiments, the gate electrode 124 is a polycrystallinesilicon (polysilicon) layer. In one or more embodiments, the polysiliconlayer is doped for proper conductivity, e.g., in a gate-first process.In one or more embodiments, the polysilicon is not necessarily dopedwhere it is a dummy gate to be replaced in a subsequent gate replacement(gate last) process. In one or more embodiments, the gate electrode 124comprises a conductive layer having a proper work function. For example,a p-type work function metal (p-metal) comprises TiN and/or TaN, whereasan n-type work function metal (n-metal) comprises Ta, TiAl, TiAlN,and/or TaCN. In one or more embodiments, the work function layercomprises doped conducting oxide materials. In one or more embodiments,the gate electrode 124 comprises other conductive materials, such asaluminum, copper, tungsten, metal alloys, metal silicide, other suitablematerials, or combinations thereof. For example, where the gateelectrode 124 comprises a work function layer, another conductive layeris formed over the work function layer.

In some embodiments, a hard mask layer is formed over the gate electrode124 to function as an etch mask, and/or to protect the underlying layersfrom damage during subsequent processing. In one or more embodiments,such hard mask layer comprises silicon oxide, silicon nitride, siliconoxynitride, silicon carbide, or combinations thereof.

The isolation feature 130 is formed at least partially in the substrate110. In some embodiments, the isolation feature 130 is a raisedstructure partially located outside the substrate 110. The isolationfeature 130 utilizes isolation technology, such as local oxidation ofsilicon (LOCOS) and/or shallow trench isolation (STI), to define andelectrically isolate various regions. The isolation feature 130comprises silicon oxide, silicon nitride, silicon oxynitride, othersuitable materials, or combinations thereof. The isolation feature 130is formed by any suitable process. In one example, the formation of anSTI comprises a photolithography process, etching a trench in thesubstrate, and filling the trench (for example, by using a CVD process)with one or more dielectric materials. In one or more embodiments, thefilled trench has a multi-layer structure, such as a thermal oxide linerlayer filled with silicon nitride or silicon oxide.

In some embodiments, an etching process is performed to remove an upperportion of the substrate 110, to ensure that bottom portions of the gatestack 120 will be fully protected by subsequently formed spacers. Insome embodiments, regions of the substrate 110 outside the gate stack120 and isolation feature 130 are doped with dopants to be lightly dopedsource/drain (LDD) regions. The dopants are selected depending on thetype of the devices to be formed in the LDD regions. For example, LDDregions for forming NMOS devices are doped with an n-type dopant, suchas phosphorous or arsenic, and LDD regions for forming PMOS devices aredoped with a p-type dopant, such as boron or BF2.

In the operation in FIG. 1B, a liner 126 and dummy spacers 128 areformed on sidewalls of the gate stack 120 to define a gate structure129. In some embodiments, the dummy spacers 128 are formed on thesidewalls of the gate stack 120 without the liner 126 which is to beformed at a later operation. The liner 126 comprises an oxide material,such as silicon oxide and/or another suitable dielectric material. Thedummy spacers 128 comprise a nitride material, such as silicon nitride,silicon oxide, silicon oxynitride, or combinations thereof. In someembodiments, the liner 126 and dummy spacers 128 are formed by blanketdepositing a first dielectric layer for defining the liner 126 over thesubstrate 110 (including the gate stack 120) and a second dielectriclayer for defining the dummy spacers 128 over the first dielectriclayer. The dielectric layers are then anisotropically etched to form theliner 126 and dummy spacers 128.

An etching process is performed to remove portions of the substrate 110outside the gate structure 129 and the isolation feature 130 to formrecesses 140 in the substrate 110. In some embodiments, the recesses 140are formed in previously formed LDD regions. The etching processcomprises a dry etching process, a wet etching process, or combinationsthereof.

In the operation in FIG. 1C, a semiconductor material is deposited inthe recesses 140 to form source/drain features 151, 152. One of thesource/drain features 151, 152 defines a source region for the NMOS orPMOS device being manufactured, and the other of the source/drainfeatures 151, 152 defines a drain region for the NMOS or PMOS devicebeing manufactured. In one or more embodiments, an epitaxial process(epitaxy) is performed to deposit the semiconductor material in therecesses 140. Examples of the epitaxial process include, but are notlimited to, a selective epitaxy growth (SEG) process, CVD depositiontechniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD(UHV-CVD)), molecular beam epitaxy, other suitable epitaxial processes,or combinations thereof. In one or more embodiments, the epitaxialprocess uses gaseous and/or liquid precursors, which interact with thecomposition of the substrate 110.

In one or more embodiments, the semiconductor material deposited in therecesses 140 comprises an appropriate stressor for enhancing chargecarrier mobility of the device being manufactured. For example, for aPMOS device, the stressor is configured to apply a compressive stress toenhance hole mobility. For an NMOS device, the stressor is configured toapply a tensile stress to enhance electron mobility. Examples of thestressor material include, but are not limited to, SiGe, SiC, GeSn,SiGeSn, and other suitable materials. In some embodiments described indetail herein below, the semiconductor material deposited in therecesses 140 comprises epitaxially grown SiGe. Similar descriptions areapplicable to other stressor materials in further embodiments.

In one or more embodiments, the source/drain features 151, 152 made ofSiGe include an upper portion having a lower concentration of Ge and alower portion having a higher concentration of Ge. For example, thesource/drain feature 151 includes an upper portion 152 having a lowerconcentration of Ge and a lower portion 153 having a higherconcentration of Ge. Likewise, the source/drain feature 152 includes anupper portion 154 having a lower concentration of Ge and a lower portion156 having a higher concentration of Ge. The upper portion 153 or 154includes a lower concentration of Ge, and hence a higher concentrationof Si, than the corresponding lower portion 155 or 156. On the one hand,the upper portion 153 or 154 with the higher concentration of Si isconfigured to form a silicide layer in a subsequent silicidation processand/or to provide appropriate etch selectivity in a subsequent contactetch operation. On the other hand, the lower portion 155, 156 with thehigher concentration of Ge is configured to provide appropriate stress,such as a tensile stress for an NMOS device or a compressive stress fora PMOS device.

In one or more embodiments, the source/drain feature 151, which isadjacent the isolation feature 130, is formed with a non-uniformthickness that is decreased from the gate stack 120 toward the isolationfeature 130. Specifically, the source/drain feature 151 includes a firstside 157 and a second side 159. The first side 157 of the source/drainfeatures 151, 152 contacts the isolation feature 130. The second side159 is father from the isolation feature 130 than the first side 157,and is adjacent the gate stack 120. The source/drain feature 151 has agreater thickness at the second side 159 than at the first side 157. Thesource/drain feature 151 is slanted downwardly from the second side 159to the first side 157. This situation is observed when the material ofthe source/drain feature 151 does not adhere well to the material of theisolation feature 130. For example, when the source/drain feature 151 ismade of SiGe which does not adhere well to SiO₂ of the isolation feature130, the source/drain feature 151 has a decreasing thickness and isslanted downwardly from the gate stack 120 to the isolation feature 130.

The source/drain feature 152 which is not adjacent the isolation feature130, is formed with a thickness which is more uniform than that of thesource/drain feature 151. The thickness of the source/drain feature 152at opposite sides thereof is about the same. In one or more embodiments,the source/drain feature 152 is formed as a raised structure projectingupwardly from the substrate 110.

In one or more embodiments, the source/drain features 151, 152 are dopedby an ion implantation process, a plasma immersion ion implantation(PIII) process, a gas and/or solid source diffusion process, othersuitable process, or combinations thereof. In one or more embodiments,the source/drain features 151, 152 are further exposed to annealingprocesses, such as a rapid thermal annealing (RTA) process.

In one or more embodiments, the dummy spacers 128 are removed after theformation of the source/drain features 151, 152. For example, the dummyspacers 128 are removed by a wet etching using phosphoric acid orhydrofluoric acid, or by a dry etching using a suitable etchant. In oneor more embodiments in which the liner 126 is not previously formed inthe operation described with respect to FIG. 1B, the liner 126 is formedafter the removal of the dummy spacers 128 to protect the sidewalls ofthe gate stack 120 from damage during subsequent operations. In one ormore embodiments, the dummy spacers 128 are not removed, and mainspacers are subsequently formed over the dummy spacers 128. The removalof the dummy spacers 128 and/or the formation of the liner 126 (whichincludes material layer deposition and etching in one or moreembodiments) are likely to result in a loss of a thickness of the upperportions 153 and 154.

In the operation in FIG. 1D, a stress memorization technology (SMT)layer 160 is formed over the substrate 110 to provide the source/drainfeatures 151, 152 with a predetermined stress, i.e., a tensile stress ora compressive stress depending on the type of the MOS device beingmanufactured. In one or more embodiments, the SMT layer 160 is blanketdeposited over the substrate 110, including the gate stack 120, theisolation feature 130, and the source/drain features 151, 152. Examplesof materials for the SMT layer 160 include, but are not limited to,nitrides, oxides, oxynitrides, tetra ethyl ortho silicate (TEOS), othersuitable materials, or combinations thereof. Examples of methods forforming the SMT layer 160 include, but are not limited to, PECVD andLPCVD.

In one or more embodiments, the substrate 110 with the SMT layer 160formed thereon is subject to an annealing process. Examples of anannealing process include, but are not limited to, RTA, furnaceannealing, flashing annealing, laser annealing, and other suitableprocesses. During the annealing process, stress is transferred from theSMT layer 160 to the underlying structure including the stressors, i.e.,the source/drain features 151, 152. As a result, after a subsequentremoval of the SMT layer 160, the stress is still retained, i.e.,memorized, by the source/drain features 151, 152, thereby enhancingcarrier mobility and performance of the device being manufactured.

In the operation in FIG. 1E, the SMT layer 160 is removed. For example,the SMT layer 160 is removed by a wet etching or a dry etching. A resistprotection oxide (RPO) layer 170 is formed over the substrate 110.Examples of materials for the RPO layer 170 include, but are not limitedto, silicon dioxide, silicon nitride, silicon oxy-nitride, oxygen-dopedsilicon nitride, nitrided oxides, other suitable materials, orcombinations thereof. The RPO layer 170 is patterned to expose silicideregions in which a silicidation process is to be performed. In one ormore embodiments, the source/drain features 151, 152 are exposed throughthe patterned RPO layer 170. In one or more embodiments, the top portionof the gate stack 120 is also exposed through the patterned RPO layer170. In one or more embodiments, the patterning of the RPO layer 170includes a wet etching, a dry etching, a plasma etching, or combinationsthereof. The removal of the SMT layer 160 and/or the patterning of theRPO layer 170 are likely to result in a further loss of an alreadythinned thickness of the upper portions 153 and 154.

A silicidation process is to be performed on the exposed silicideregions in order to reduce contact resistance. For example, such asilicidation process is performed by depositing a metal layer over thesubstrate, performing an annealing process to cause metal atoms in themetal layer to react with silicon atoms in the exposed silicide regionsto form a metal silicide, and then removing non-reacted portions of themetal layer. However, in some situations, the upper portions 153, 154 ofthe source/drain features 151, 152, which are intended to react with anoverlying metal layer to form metal silicide, are lost (removed)partially or wholly as a result of numerous operations between theformation of the source/drain features 151, 152 and the silicidationprocess. The thickness of the upper portions 153, 154 in particular, andthe thickness of the source/drain features 151, 152 in overall, becomezero or too thin, especially in an upper portion 171 of the source/drainfeature 151 on the second side 159 thereof. If a silicidation process isdirectly performed over the upper portion 171 of the source/drainfeature 151, the metal atoms are likely to penetrate the upper portion171 of the source/drain feature 151 to react with Si atoms of thesubstrate 110. This “piping” defect, schematically designated at arrowP, is further likely to cause current leakage. Further, the thinned orzero thickness of the upper portions 153, 154 of the source/drainfeatures 151, 152 reduces etch selectivity of the source/drain features151, 152 which, in turn, is likely to result in contact-etch-throughdefects.

In the operation in FIG. 1F, before performing a silicidation process, aSi-containing layer 180 is formed over the thinned source/drain features151, 152. Over the source/drain feature 151, the Si-containing layer 180follows the shape of the source/drain feature 151 and is slanteddownwardly from the second side 159 toward the first side 157 of thesource/drain feature 151, i.e., from the gate stack 120 toward theisolation feature 130. In one or more embodiments, the Si-containinglayer 180 includes a Si layer selectively grown over the source/drainfeatures 151, 152, e.g., by LPCVD. Other materials for the Si-containinglayer 180 include, but are not limited to, SiC; GeSn; SiGeSn and othersuitable materials.

After the formation of the Si-containing layer 180, a metal layer 185 isformed over the substrate 110, and covers the Si-containing layer 180.An annealing process is then performed to cause metal atoms in the metallayer 185 to react with Si atoms in the Si-containing layer 180 to forma metal silicide layer 190 as shown in FIG. 1G. Un-reacted portions ofthe metal layer 185, which do not overlie the source/drain features 151,152, are selectively removed, e.g., via a wet etching. In one or moreembodiments, the metal layer includes at least one of Pt, W, Ni, Ti, orCo deposited, e.g., by PVD.

In some embodiments, the Si-containing layer 180 and metal layer 185 arealso formed over the gate stack 120. As a result, the metal silicidelayer 190 is also formed over the gate stack 120, thereby improving thegate electrode contact resistance. In one or more embodiments, theSi-containing layer 180 is completely consumed during the silicidationprocess. In one or more embodiments, the Si-containing layer 180 ispartially consumed during the silicidation process.

In one or more embodiments, the Si-containing layer 180 is configured toprevent metal atoms in the metal layer 185 from penetrating thesource/drain features 151, 152 to react with Si atoms in the substrate110. For example, the Si-containing layer 180 is a selectively grown Silayer having a thickness from 10 to 20 nm. Because metal atoms in themetal layer 185 are prevented from penetrating the source/drain features151, 152 to react with Si atoms in the substrate 110, piping defects arereduced or prevented even in regions where the thickness of thesource/drain features 151, 152 in overall and/or the thickness of theupper portions 153, 154 of the source/drain features 151, 152 inparticular are low. For example, piping defects are reduced or preventedin at least the upper portion 171 of the source/drain feature 151.

Further, Si atoms of the Si-containing layer 180 replenish those thatwere lost (removed) from the upper portions 153, 154 of the source/drainfeatures 151, 152 during various operations prior to the silicidationprocess. As a result, the metal silicide layer 190 of sufficient qualityis formed to both enhance the device performance and to prevent or atleast minimize a likelihood of contact-etch-through defects which, inturn, increases the production yield.

In some embodiments, further operations are performed after thesilicidation process. For example, the RPO layer 170 is removed and mainspacers are formed on the sidewalls of the gate stack 120. In one ormore embodiments, the main spacers are formed by blanket depositing adielectric layer over the substrate 110, and then, anisotropicallyetching the dielectric layer to form the main spacers. The spacers 252are positioned on the sidewalls of the gate structures 220 and 221, andin the depicted embodiment, adjacent the dummy spacers 128. The mainspacers comprise a dielectric material, such as silicon nitride, siliconoxide, silicon oxynitride, other suitable materials, or combinationsthereof. In one or more embodiments, the RPO layer 170 is maintainedpartially or wholly.

In some embodiments, a contact etch stop layer (CESL) is formed over atop surface of the gate stack 120. Then, an inter-level dielectric (ILD)layer is formed over the substrate 110. A chemical mechanical polishing(CMP) process is performed to planarize the substrate. Contact vias areformed through the ILD layer to contact with the gate stack 120 and thesource/drain features 151, 152 via the metal silicide layer 190.Contact-etch-through defects are unlikely to occur for the reasonsdiscussed above. A multilayer interconnection (MLI) including metallayers and inter-metal dielectric (IMD) layers is formed over thesubstrate 110 to electrically connect the contact vias with variousfeatures or structures of the semiconductor device.

In one or more embodiments implementing the gate first approach, thegate electrode 124 remains in the final device. In one or moreembodiments implementing the gate last approach, the polysilicon of thegate electrode 124 is removed and replaced with a metal.

FIG. 2 is a flow chart of a method 200 of manufacturing a semiconductordevice in accordance with some embodiments.

At an operation 210, a gate stack and/or an isolation feature is/areover/in a substrate. For example, the gate stack 120 and/or theisolation feature 130 is/are over/in the substrate 110, as describedwith respect to FIG. 1A.

At an operation 220, dummy spacers are formed on sidewalls of the gatestack. For example, the dummy spacers 128 are formed on the sidewalls ofthe gate stack 120, as described with respect to FIG. 1B.

At an operation 230, a source/drain feature is formed over thesubstrate. For example, the source/drain feature 151 is formed over thesubstrate 110, as described with respect to FIG. 1C.

At an operation 240, the dummy spacers are removed and, a liner isformed on the sidewalls of the gate stack. For example, the dummyspacers 128 are removed, and the liner 126 is formed on the sidewalls ofthe gate stack 120, as described with respect to FIG. 1C.

At an operation 250, an SMT layer is formed over the substrate,annealed, and then removed. For example, the SMT layer 160 is formedover the substrate 110, annealed, and then removed, as described withrespect to FIG. 1D.

At an operation 260, an RPO layer is formed over the substrate to definesilicide regions. For example, the RPO layer 170 is formed over thesubstrate 110 to define silicide regions, i.e., the source/drainfeatures 151, 152 and the top of the gate stack 120, as described withrespect to FIG. 1E.

At an operation 270, a Si layer is selectively grown over thesource/drain feature and gate stack. For example, the Si layer isselectively grown over the source/drain feature 151, 152 and the gatestack 120, as described with respect to FIG. 1F.

At an operation 280, a metal silicide layer is formed using Si atoms ofthe Si layer and metal atoms of a meal layer. For example, the metalsilicide layer 190 is formed using Si atoms of the Si layer 180 andmetal atoms of the meal layer 185, as described with respect to FIG. 1G.

The method described herein in accordance with some embodiments isuseful for manufacturing semiconductor devices using technology nodes at40 nm and below. The method described herein in accordance with someembodiments is also useful for manufacturing semiconductor devices usingtechnology nodes above 40 nm.

The above method embodiment shows exemplary operations, but they are notnecessarily required to be performed in the order shown. Operations maybe added, replaced, changed order, and/or eliminated as appropriate, inaccordance with the spirit and scope of embodiments of the disclosure.Embodiments that combine different features and/or different embodimentsare within scope of the disclosure and will be apparent to those skilledin the art after reviewing this disclosure.

One aspect of this description relates to a semiconductor device. Thesemiconductor device includes an isolation feature in a substrate. Thesemiconductor device further includes a first source/drain feature inthe substrate, wherein a first side of the first source/drain featurecontacts the isolation feature, and the first source/drain featureexposes a portion of the isolation feature below a top surface of thesubstrate. The semiconductor device further includes a silicide layerover the first source/drain feature. The semiconductor device furtherincludes a dielectric layer along the exposed portion of the isolationfeature below the top surface of the substrate, wherein the dielectriclayer contacts the silicide layer. The semiconductor device furtherincludes a second source/drain feature in the substrate on an oppositeside of a gate stack from the first source/drain feature, wherein thesecond source/drain feature has a substantially uniform thickness.

Another aspect of this description relates to a semiconductor device.The semiconductor device includes an isolation feature in a substrate.The semiconductor device further includes a first source/drain featurein the substrate, wherein a first side of the first source/drain featurecontacts the isolation feature, and a first portion of the firstsource/drain feature adjacent to isolation feature has a differentthickness from a second portion of the first source/drain featuredistant from the isolation feature. The semiconductor device furtherincludes a silicide layer over the first source/drain feature. Thesemiconductor device further includes a dielectric layer along asidewall of the isolation feature of the isolation feature below a topsurface of the substrate. The semiconductor device further includes asecond source/drain feature in the substrate on an opposite side of agate stack from the first source/drain feature, wherein the secondsource/drain feature has a substantially uniform thickness.

Still another aspect of this description relates to a method of forminga semiconductor device. The method includes etching a substrate todefine a plurality of trenches in a substrate, wherein a first trench ofthe plurality of trenches exposes a sidewall of an isolation feature inthe substrate. The method further includes depositing a firstsource/drain feature in the first trench of the plurality of trenches,wherein a first side of the first source/drain feature contacts theisolation feature, and the first source/drain feature exposes a portionof the isolation feature below a top surface of the substrate. Themethod further includes siliciding the first source/drain feature toform a silicide layer. The method further includes forming a dielectriclayer over the substrate, wherein the dielectric layer contacts theexposed portion of the isolation feature. The method further includesdepositing a second source/drain feature in a second trench of theplurality of trenches, wherein the second source/drain feature has asubstantially uniform thickness.

It will be readily seen by one of ordinary skill in the art that one ormore of the disclosed embodiments fulfill one or more of the advantagesset forth above. After reading the foregoing specification, one ofordinary skill will be able to affect various changes, substitutions ofequivalents and various other embodiments as broadly disclosed herein.It is therefore intended that the protection granted hereon be limitedonly by the definition contained in the appended claims and equivalentsthereof.

What is claimed is:
 1. A semiconductor device, comprising: an isolationfeature in a substrate; a first source/drain feature in the substrate,wherein a first side of the first source/drain feature contacts theisolation feature, and the first source/drain feature exposes a portionof the isolation feature below a top surface of the substrate; asilicide layer over the first source/drain feature; a dielectric layeralong the exposed portion of the isolation feature below the top surfaceof the substrate, wherein the dielectric layer contacts the silicidelayer; and a second source/drain feature in the substrate on an oppositeside of a gate stack from the first source/drain feature, wherein thesecond source/drain feature has a substantially uniform thickness. 2.The semiconductor device of claim 1, wherein a top surface of theisolation feature is substantially coplanar with the top surface of thesubstrate.
 3. The semiconductor device of claim 1, wherein a thicknessof the first source/drain feature adjacent to the gate stack is greaterthan a thickness of the first source/drain feature adjacent to theisolation feature.
 4. The semiconductor device of claim 1, wherein thedielectric layer extends along a sidewall of the gate stack.
 5. Thesemiconductor device of claim 1, wherein the first source/drain featurecomprises a top portion and a bottom portion, wherein the bottom portionhas a different composition from the top portion.
 6. The semiconductordevice of claim 5, wherein the dielectric layer contacts the top portionof the first source/drain feature.
 7. The semiconductor device of claim5, wherein the top portion of the first source/drain feature separatesthe dielectric layer from the bottom portion of the first source/drainfeature.
 8. The semiconductor device of 1, wherein the dielectric layerextends along a top surface of the isolation feature.
 9. Thesemiconductor device of claim 1, wherein the dielectric layer contactsthe top surface of the substrate.
 10. A semiconductor device,comprising: an isolation feature in a substrate; a first source/drainfeature in the substrate, wherein a first side of the first source/drainfeature contacts the isolation feature, and a first portion of the firstsource/drain feature adjacent to isolation feature has a differentthickness from a second portion of the first source/drain featuredistant from the isolation feature; a silicide layer over the firstsource/drain feature; a dielectric layer along a sidewall of theisolation feature of the isolation feature below a top surface of thesubstrate; and a second source/drain feature in the substrate on anopposite side of a gate stack from the first source/drain feature,wherein the second source/drain feature has a substantially uniformthickness.
 11. The semiconductor device of claim 10, wherein thedielectric layer separates the silicide layer from the isolationfeature.
 12. The semiconductor device of claim 10, wherein a dimensionof the silicide layer in a first direction parallel to the top surfaceof the substrate is less than a dimension of the first source/drainfeature in the first direction.
 13. The semiconductor device of claim10, wherein the dielectric layer contacts a top surface of the isolationfeature.
 14. The semiconductor device of claim 10, further comprising agate stack, wherein the dielectric layer extends along a sidewall of thegate stack.
 15. The semiconductor device of claim 14, wherein thedielectric layer is between the silicide layer from the gate stack. 16.The semiconductor device of claim 10, wherein a top surface of thesilicide layer adjacent to the dielectric layer is below the top surfaceof the substrate.
 17. A method of forming a semiconductor device, themethod comprising: etching a substrate to define a plurality of trenchesin a substrate, wherein a first trench of the plurality of trenchesexposes a sidewall of an isolation feature in the substrate; depositinga first source/drain feature in the first trench of the plurality oftrenches, wherein a first side of the first source/drain featurecontacts the isolation feature, and the first source/drain featureexposes a portion of the isolation feature below a top surface of thesubstrate; siliciding the first source/drain feature to form a silicidelayer; forming a dielectric layer over the substrate, wherein thedielectric layer contacts the exposed portion of the isolation feature;and depositing a second source/drain feature in a second trench of theplurality of trenches, wherein the second source/drain feature has asubstantially uniform thickness.
 18. The method of claim 17, furthercomprising patterning the dielectric layer to expose a portion of thefirst source/drain feature.
 19. The method of claim 18, whereinsiliciding the first source/drain feature comprises siliciding theportion of the first source/drain feature exposed by patterning thedielectric layer.
 20. The method of claim 17, wherein forming thedielectric layer comprises forming the dielectric layer on sidewalls ofa gate stack between the first trench of the plurality of trenches andthe second trench of the plurality of trenches.